The present Invention relates, in general, to a novel coherent anti-Stokes Raman spectroscopy (CARS) method and to a novel apparatus for carrying out this method.
Methods of obtaining broadband, or multiplexed, coherent anti-Stokes Raman spectroscopy (herein often referred to as CARS) spectra in a single laser shot have previously been accomplished on the nanosecond time scale. These methods have used untuned broadband dye laser sources to provide a continuous band of visible Stokes frequencies for the CARS four-wave mixing process. However, the untuned dye lasers have outputs extending over only several hundred wave numbers for a particular dye used and do not lend themselves readily to subnanosecond operation.
The ability to perform optical spectroscopy in the picosecond range is highly desirable in order to observe dynamical phenomena occuring on a time scale of picoseconds. Picosecond applications of CARS have to date only been carried out with the use of narrow-band tunable Stokes frequencies, thus necessitating many laser shots to build up a point-by-point spectrum. Such approaches are not practical for acquiring extensive vibrational spectra of transient, non-repetitive events.
Picosecond white-light continuum pulses have previously been used in optical spectroscopy including a method called inverse Raman scattering. An example of this process is described in U.S. Pat. No. 3,782,828 to ALFANO et al. Inverse Raman scattering, while also providing broadband vibrational spectra, is a Raman absorption process which is distinctly different in approach and applicability from the CARS emission process.